The present invention relates generally to MR imaging and, more particularly, to a method and apparatus of gradient echo imaging with on-the-fly optimization of tissue suppression.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
To enhance a radiologist or other medical provider's ability to efficiently and effectively diagnose a particular pathology or abnormality, it is often desirable to suppress signal from certain tissues while simultaneously obtaining signal from other tissues. A number of data acquisition schemes have been developed for suppressing specific tissues. This method of tissue suppression can also eliminate signal based not only on frequency selection, but T1. For example, one tissue suppression imaging technique implements a frequency selective saturation pulse that is applied before standard imaging pulses of a sequence. As a result, the saturation pulse sets to zero the magnetization of the particular tissue to be suppressed, e.g. fat. As such, when the standard imaging sequence is applied, no signal from the suppressed component or tissue is detected. Accordingly, the non-detected components will appear black on a reconstructed image and thereby provide contrast to the detected and imaged components or tissue. The tissue suppression techniques may be implemented with spin echo imaging as well as gradient echo imaging. For gradient echo imaging sequences, which are designed for rapid imaging, use of standard tissue suppression techniques dramatically increases acquisition time. This negatively affects patient throughput and increases the likelihood of patient movement during the imaging process which negatively affects the reconstructed image.
One proposed solution is to acquire several lines of k-space following each application of the tissue suppression technique. For this approach to be successful, the signal from the suppressed tissue must remain null for some time after the suppression technique is applied. The time and duration for which the suppressed tissue remains null after the saturation pulse varies with tissue T1 and imaging parameters, i.e. resolution, receiver bandwidth, flip angle, TR, and the like. Despite the dependence of suppression time upon imaging parameters, known imaging techniques maintain a timing of the suppression pulses and k-space filling scheme regardless of the particular imaging parameters. This results in an unpredictable quality of tissue suppression and image artifacts.
It would therefore be desirable to have a system and method capable of setting, on-the-fly, timing of a series of suppression pulses as well as a k-space filling scheme that is optimized to fit particular user-prescribed imaging parameters for an imminent scan.